The present invention relates to hydrostatic transmissions and particularly to a tramsmission whose output drive is optimally dervied from the selective combination of plural hydrostatic outputs.
Pure hydrostatic transmissions typically utilize a single mechanically driven hydraulic pump connected in hydraulic fluid circuit with a single hydraulic motor to develop a hydrostatic transmission output which, by varying the displacement of one or both of these hydraulic devices, can be infinitely variable in speed. The hydraulic capacities, i.e. , maximum displacements, of the pump and motor are determined by the torque and speed requirements of a particular load. To satisfy high load torque demands, their hydraulic capacities must be large, and thus the pump and motor must both be of large physical size. As such, their hydraulic and mechanical losses are significant. Then, at high output speeds, the centrifugal forces and moments of a large hydraulic motor rotating at high speeds become a serious problem.
To alleviate the problems associated with high capacity hydraulic pump and motor hydrostatic drive units, hydromechanical drives or transmissions have been developed and are now in wide use. Transmissions of this type utilize a mechanical input from a prime mover which is combined with the hydrostatic output from a prime mover-driven hydraulic driven unit in a shiftable cdombining gear set to develop a multiple-range, infinitely variable hydromechanical output for driving a load.
While hydromechanical transmissions are well suited for a wide variety of applications, they are relatively complex and costly to manufacture. They futher require a mechanical drive connection from the prime mover to the transmission output, while a hydrostatic transmission can use oil lines which are more adaptable to complex transmission configurations. Moreover, certain applications do not required the versatility and high speed propulsion afforded by hydromechanical transmissions, and thus, for these applications, their expense is not justified.
It is accordingly an object of the present invention to provide an improved hydrostatic transmission.
A further object is to provide a hydrostatic transmission of the above-character which has enhanced operating efficiencies.
An additional object is provide a hydrostatic transmission of the above-character which has improved operating performance.
Yet another object is to provide a hydrostatic transmission of the above-character which possesses improved torque-speed operating characteristics.
Another object is to provide a hydrostatic transmission of the above-character which is compact in size and light in weight.
A still further object is to provide a hydrostatic transmission of the above character which is inexpensive to manufacture, readily adaptable to a variety of load applications and reliable over a long operating life.
Other objects of the invention will in part be obvious and in part appear hereinafter.